A conventional air compressor, as shown in FIG. 6, is disclosed in a publication entitled "Reading-Book of Air-Pressure for Engineering Development" published by KAIHATSU-sha on Mar. 10, 1971. Referring to FIG. 6, the air compressor described in that publication comprises a housing 100 having a cylinder 101, a piston 102 movably located in the cylinder 101, an intake valve 103 functioning as a one-way valve, and an exhaust valve 106 functioning as a one-way valve. The intake valve 103 opens during a suction process of the air compressor and permits air flow from the atmosphere into a compression space 104 formed in the cylinder 101 through a filter 105. The exhaust valve 106 opens during a discharge process of the air compressor and permits air flow from the compression space 104 through a pipe 111 to an apparatus which uses the air pressure. Both of the intake valve 103 and the exhaust valve 106 are located in a cylinder head 114.
The piston 102 is connected to a crank shaft 108 by way of a connecting rod 107. The crank shaft 108 is located in a crank space 109 and is connected to a drive source (not shown). The connecting rod 107 transforms revolutionary movement of the crank shaft 108 into axial movement of the piston 102. That is, the piston 102 moves upwardly and downwardly in the cylinder 101. Such movement of the piston 102 periodically increases and decreases the volume in the compression space 104, and air sucked through the filter 105 and the intake valve 103 is compressed and discharged through the exhaust valve 106 by way of the pipe 111.
The open area of the intake valve 103 and the exhaust valve 106 cannot be enlarged since each valve 103, 106 is located on one side of the piston 102 in the cylinder head 114. Therefore, each open area of the intake valve 103 and the exhaust valve 106 must be small and the small open areas of the valves 103, 106 resists air flow through the valves 103, 106. As a result of the resistance to air flow, the discharged air flow of the air compressor becomes low during low revolutionary speed and high revolutionary speed as shown by line C in FIG. 5.
Further, not only the volume in the compression space 104 but also the volume in the crank space 109 is increased and decreased according to the movement of the piston 102 so that the temperature in the crank space 109 rises. A cooling fan 110 is thus required for cooling the crank space 109.